It is different from a TDMA or CDMA system, in a TD-CDMA system each time-slot includes several code channels, which are used to distinguish different subscribers in the same time-slot. Applying hybrid TDMA and CDMA mode has performance advantage than the only TDMA or CDMA mode. In a TD-CDMA system, a receiver not only needs to separate different time-slots in each frame, but also needs to separate different code channels in the same time-slot with correlation methods. With these methods, different channels can be differentiated at the receiving end. After transmission, a signal, sent by the transmitting end of a system, will be put in interference by the space time-variant channel, where the signal passes through. In addition, multipath interference of a mobile channel and the channel itself also causes intersymbol interference (ISI) and multiple access interference (MAI). In order to recover correctly a transmitted signal at the receiving end, it is necessary to obtain the channel impulse response. With the received signal and the channel impulse response, a transmitted signal will be correctly estimated. Therefore, quality of channel estimation occupies a pivotal position for a system performance.
In a modern wireless communication system, because of multipath fading and time delay spread a received digital signal, which has been passed through a radio channel, has frequency selective fading, time selective fading, wave distortion and lower signal-to-noise ratio. Therefore, it is difficult to recover a transmitted signal at the receiving end. In many systems, training sequence (called preamble code or midamble code, according to their position) is used to estimate the channel. Obviously, generating method of a training sequence is very important.
Reference [1], Bernd Steiner and Peter Jung: “Uplink Channel Estimation in Synchronous CDMA Mobile Radio Systems with Joint Detection”, PIMRC'93, discloses a maximum likelihood non-deviated channel estimation method and a matching filter deviated channel estimation method. At the same time, reference [1] has provided a training sequence generating method, which considers that training sequences for different subscribers in a same time-slot are based on a same basic code or cycle basic code, with different time shifting each other. In reference [1], it is supposed that channel impulse response of each subscriber should have same length, so relative time shifting of training sequence code of each subscriber is an equal interval.
For the present technology as shown in reference [1], FIG. 1 and FIG. 2 show diagrams of different subscribers training sequences which are obtained by shifting the basic code sequentially. Wherein the offset of generated training sequence from the basic code corresponds to the estimated window length. In this case, whether the real subscriber number reaches the maximum subscriber number, estimated window length of each subscriber all selects W, i.e. corresponding to the maximum subscriber number situation.
In FIG. 1, the basic code is non-cyclic. The estimated window length is W = P/K, wherein K is the maximum subscribers number working simultaneously, and P is length of the basic code. In FIG. 2, the basic code is cyclic. The estimated window length is W = P/K, wherein K is the maximum subscribers number working simultaneously, and P is period of the basic code. When selecting cyclic basic code, processing complexity of channel estimation at receiving end will be simplified. For method of the invention, two basic code cases are no difference in principle.
The generating methods of a training sequence from a basic code, mentioned above, are all supposed that channel impulse response of every subscriber has the same length, and relative time shift between training sequences of every subscriber has equal interval. This kind of generating methods cannot fully develop the channel estimation advantage, so it is unavoidable that the communication system performance will be affected.